1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having a mechanism which can control the amount of a material removed from a peripheral portion of the workpiece during a polishing operation.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. This process is called Chemical Mechanical polishing.
If the semiconductor wafer is not pressed against the polishing cloth under forces which are uniform over the entire surface of the semiconductor wafer, then the semiconductor wafer tends to be polished insufficiently or excessively in local areas depending on the applied forces. The following arrangements have been proposed in the art to prevent the semiconductor wafer from being pressed against the polishing cloth under irregular forces.
1) One conventional solution has been to apply an elastic pad of polyurethane or the like to a workpiece holding surface of the top ring for uniformizing a pressing force applied from the top ring to the semiconductor wafer.
2) According to another solution, the top ring, i.e., a workpiece carrier for holding a semiconductor wafer, is tiltable with respect to the surface of the polishing cloth.
3) Still another attempt has been to press a region of the polishing cloth surrounding the semiconductor wafer, independently of the semiconductor wafer, for thereby eliminating an appreciable step between a region of the polishing cloth pressed by the semiconductor wafer and the surrounding region thereof.
FIG. 8 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 8, the conventional polishing apparatus comprises a turntable 41 with a polishing cloth 42 attached to an upper surface thereof, a top ring 45 for holding a semiconductor wafer 43 to press the semiconductor wafer 43 against the polishing cloth 42, and an abrasive liquid supply nozzle 48 for supplying an abrasive liquid Q to the polishing cloth 42. The top ring 45 is connected to a top ring shaft 49, and is provided with an elastic pad 47 of polyurethane or the like on its lower surface. The semiconductor wafer 43 is held by the top ring 45 in contact with the elastic pad 47. The top ring 45 also has a cylindrical presser ring 46A on an outer circumferential edge thereof for retaining the semiconductor wafer 43 on the lower surface of the top ring 45. Specifically, the presser ring 46A is fixed to the top ring 45, and has a lower end projecting downwardly from the lower surface of the top ring 45 for holding the semiconductor wafer 43 on the elastic pad 47 against removal off the top ring 45 under frictional engagement with the polishing cloth 42 during a polishing process.
In operation, the semiconductor wafer 43 is held against the lower surface of the elastic pad 47 which is attached to the lower surface of the top ring 45. The semiconductor wafer 43 is then pressed against the polishing cloth 42 on the turntable 41 by the top ring 45, and the turntable 41 and the top ring 45 are rotated independently of each other to move the polishing cloth 42 and the semiconductor wafer 43 relatively to each other, thereby polishing the semiconductor wafer 43. The abrasive liquid Q comprises an alkaline solution containing an abrasive grain of fine particles suspended therein, for example. The semiconductor wafer 43 is polished by a composite action comprising a chemical polishing action of the alkaline solution and a mechanical polishing action of the abrasive grain.
FIG. 9 of the accompanying drawings shows in a fragmental cross-section the semiconductor wafer 43, the polishing cloth 42, and the elastic pad 47. As shown in FIG. 9, the semiconductor wafer 43 has a peripheral portion which is a boundary between contact and noncontact with the polishing cloth 42 and also is a boundary between contact and noncontact with the elastic pad 47. At the peripheral portion of the semiconductor wafer 43, the polishing pressure applied to the semiconductor wafer 43 by the polishing cloth 42 and the elastic pad 47 is not uniform, thus the peripheral portion of the semiconductor wafer 43 is liable to be polished to an excessive degree. As a result, the peripheral edge of the semiconductor wafer 43 is often polished in a so-called "edge-rounding" manner.
In order to prevent the peripheral portion of the semiconductor wafer from being excessively polished, there has been proposed in Japanese patent application No. 8-54055 a polishing apparatus having a structure for pressing an area of the polishing cloth which is located around the peripheral portion of the semiconductor wafer.
FIG. 10 of the accompanying drawings shows the polishing apparatus disclosed in Japanese patent application No. 8-54055. As shown in FIG. 10, a semiconductor wafer 43 is held by a top ring 45 and pressed against a polishing cloth 42 on a turntable 41. The semiconductor wafer 43 is retained on the top ring 45 by a cylindrical retaining portion extending downwardly from the top ring 45. A presser ring 46 is disposed around and connected to the top ring 45 by keys 58. The keys 58 allow the presser ring 46 to move vertically with respect to the top ring 45 and to rotate together with the top ring 45. The presser ring 46 is rotatably supported by a radial bearing 59 which is held by a bearing holder 60 operatively coupled by a plurality of (e.g. three) circumferentially spaced shafts 61 to a plurality of (e.g. three) circumferentially spaced presser ring air cylinders 62. The presser ring air cylinders 62 are fixedly mounted on a top ring head 69. The top ring 45 has an upper surface held in sliding contact with a spherical bearing 65 that is slidably supported on the lower end of a top ring shaft 66. The top ring shaft 66 is rotatably supported by the top ring head 69. The top ring 45 is vertically movable by a top ring air cylinder 67 mounted on the top ring head 69 and operatively connected to the top ring shaft 66.
The top ring air cylinder 67 and the presser ring air cylinders 62 are connected to a compressed air source 64 respectively through regulators R1 and R2. The regulator R1 regulates the air pressure supplied from the compressed air source 64 to the top ring air cylinder 67 to adjust the pressing force for pressing the semiconductor wafer 43 against the polishing cloth 42 by the top ring 45. The regulator R2 regulates the air pressure supplied from the compressed air source 64 to the presser ring air cylinders 62 to adjust the pressing force for pressing the presser ring 46 against the polishing cloth 42. By adjusting the pressing force of the presser ring 46 with respect to the pressing force of the top ring 45, the distribution of polishing pressures is made continuous and uniform from the center of the semiconductor wafer 43 to its peripheral edge and further to the outer circumferential edge of the presser ring 46 disposed around the semiconductor wafer 43. Consequently, the peripheral portion of the semiconductor wafer 43 is prevented from being polished excessively or insufficiently.
In the polishing apparatus proposed in Japanese patent application No. 8-54055, the top ring 45 and the presser ring 46 are integrally rotated, thus there occurs no relative rotation between the semiconductor wafer 43 held by the lower surface of the top ring 45 and the presser ring 46. Therefore, the polishing is performed in such a state that the outer circumferential edge of the semiconductor wafer 43 and the inner circumferential surface of the presser ring 46 are always in confrontation with each other at the same portions or areas.
However, the pressing surface, i.e., the lower end surface of the presser ring 46 is not necessarily flat microscopically, and has undulations or irregularities, and hence there occurs a small difference locally in deformation of the polishing cloth to lead to nonuniform deformation of the polishing cloth around the semiconductor wafer. This nonuniform deformation of the polishing cloth affects the amount of the material removed from the peripheral portion of the semiconductor wafer, and the entire peripheral portion of the semiconductor wafer cannot be polished uniformly. Further, since the presser ring does not have uniform vertical thickness in an entire circumference, the entire peripheral portion of the semiconductor wafer also cannot be polished uniformly.
Further, in the polishing apparatus disclosed in Japanese patent application No. 8-54055, by pressing a wide area of the polishing cloth around the peripheral portion of the semiconductor wafer by the presser ring, the distribution of applied polishing pressures, which result from a combination of the pressing forces exerted by the top ring and the presser ring, is continuous and uniform from the center of the semiconductor wafer to its peripheral edge and further to an outer circumferential edge of the presser ring. Therefore, the presser ring is required to have a relatively large radial thickness, providing a relatively large surface area on its lower pressing surface. Insofar as the surface of the polishing cloth and the lower surface of the presser ring lie parallel to each other, no problem arises. However, if the surface of the polishing cloth and the lower surface of the presser ring are brought out of parallelism with each other due to undulations or irregularities of the surface of the polishing cloth, then only an outer peripheral portion of the presser ring 46 presses the polishing cloth 42 as shown in FIG. 11 of the accompanying drawings. When the condition of the polishing cloth shown in FIG. 11 occurs, the polishing cloth 42 tends to rise near the peripheral portion of the semiconductor wafer 43, and hence the peripheral portion of the semiconductor wafer 43 is liable to be polished to an excessive degree, thus causing edge-rouding.
The top ring 45 needs to provide a downwardly open recess in its lower surface for holding the semiconductor wafer 43 therein. Such a downwardly open recess may be formed by an outer circumferential wall extending downwardly integrally from the top ring 45 or an annular retainer ring fixedly provided around the top ring 45. If the top ring 45 is made of ceramics, then it is not practical to provide the top ring 45 with such a downwardly extending outer circumferential wall from the viewpoint of machining or production cost. Another way of providing a downwardly open recess in the lower surface of the top ring 45 is to secure a retainer ring 50 around the top ring 45, as shown in FIG. 11. With the retainer ring 50 interposed between the outer circumferential edge of the semiconductor wafer 43 and the presser ring 46, the distance between the inner circumferential edge of the presser ring 46 and the outer circumferential edge of the semiconductor wafer 43 is so large that the presser ring 46 fails to press the polishing cloth 42 near the outer circumferential edge of the semiconductor wafer 43. As a result, the polishing cloth 42 tends to rise near the outer circumferential edge of the semiconductor wafer 43 which is then excessively polished into an edge-rounding.